The present invention relates to a projection lens unit, comprising a projection lens system which enlarges and projects image light from an image formation device onto a screen and a lens barrel which holds lenses of the projection lens system. In particular the present invention relates to correction of the focal point of the projection lens system when the temperature rises in the projection lens unit.
As projection-type image display devices, projectors comprising DMDs (Digital Micromirror Devices) and other reflection-type image formation devices, and which enlarge and project onto a screen an image appearing on the image formation device by means of a projection lens system comprising a plurality of lenses, are well-known. Because in large-size projectors for use in cinemas, which project images onto large-size screens, a high-output light source is used for illumination of the reflection-type image formation device, the increase in temperature of the projection lens system is significant. The increase in temperature is particularly significant for that portion of the projection lens system on the side closer to the image formation device. An increase in temperature causes a change in the refractive index of the materials comprised by lenses as well as thermal expansion of components comprising lenses, and it is known that, as a result, the focal point of the projection lens system may change, degrading the quality of the image projected onto the screen.
In Japanese Patent Laid-open No. 7-209609 (FIGS. 7 and 9), a configuration is disclosed for correcting for fluctuations in the focal point of a projection lens system arising from increases in temperature. Specifically, one lens is held so as to be movable with respect to the lens barrel in the optical axis direction, and by moving the lens according to increases in temperature, the focal paint is corrected. Two main aspects are disclosed in Japanese Patent Laid-open No. 7-209609 (FIGS. 7 and 9). In the first aspect, a flange is provided on the outer edge of the lens, and the lens is made to move in the optical axis direction through thermal expansion of this flange. In the second aspect, the lens is made to move in the optical axis direction by a thermal-expansion member which intervenes between the flange and the lens barrel.
However, the configuration disclosed in Japanese Patent Laid-open No. 7-209609 (FIGS. 7 and 9) has the following problems. First, the lens also expands in the radial direction with increases in temperature, and so if there is a difference in the thermal expansion coefficients of the lens and the lens barrel, the lens may dig into the lens barrel, so that the desired amount of movement is not obtained, and there is also the concern that the lens may be deformed. Further, if the gap between lens barrel and lens is set to be large in order to prevent such digging-in, then the lens inclination and eccentricity are increased. Also, complex irregular shape lens comprising a flange on the outer periphery are difficult and expensive to manufacture. In particular, it is difficult to provide a flange in the case of a glass lens. And, in the configuration of Japanese Patent Laid-open No. 7-209609 (FIGS. 7 and 9), only one lens can be moved among the plurality of lenses comprised by the projection lens system. Thus the configuration disclosed in Japanese Patent Laid-open No. 7-209609 FIGS. 7 and 9) makes effective correction difficult, and entails a complex construction.
Because high image quality is required from the above-described projector for cinema use, such a projector has an anomalous dispersion lens with positive power having excellent characteristics for correction of chromatic aberration of magnification, positioned on the image formation device side of the diaphragm of the projection lens system. An anomalous dispersion lens having this positive power has large expansion and change in refractive index accompanying increases in temperature, and so is a major cause of fluctuations in the focal point of the projection lens system with rising temperature. However, Japanese Patent Laid-open No. 7-209609 (FIGS. 7 and 9) is related to the projection lens system of a projection-type image display device having an anomalous dispersion lens with positive power which is on the image formation device side of the diaphragm, and gives no instructions relating to a configuration to optimize corrections of the focal point accompanying increases in temperature.
An object of this invention is to effectively correct changes in the focal point of a projection lens system accompanying increases in temperature, by means of a comparatively simple configuration.